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ASA dropping legitimate returning Internet traffic

Hi all,

I recently turned on on the 'stateful' feature on our Internet ASA's. It was once disabled completely due to an Asymmetric routing issue that existed within our network. I have managed to turn this back on for all but the subnets that require an Asymmetric routing path.

Now it seems that the ASA's are dropping a significant amount of packets on the OUTSIDE interface for legitimate returning TCP traffiic. At first glance I thought we were under some sort of attack; where the attacker was reversing the source and destination ports; but the source IP addresses are all legitimate well known websites. There does seem to be a ever so slight performance decrease when surfing the web.

I suspect this has something to do with TCP timeout values on either our downstream Proxy (Blue Coat PRoxySG900) or the ASAs but as far as I know they are both set to there defaults.

The proxy is masquerading the client's IP behind it's IP Address ( and proxy is natted behind the ASA's OUTSIDE Interface IP Address.

My Syslog Server is being flooded with messages like the ones below;

%ASA-4-106100: access-list outside_access_in denied tcp OUTSIDE/ -> INSIDE/

%ASA-4-106100: access-list outside_access_in denied tcp OUTSIDE/ -> INSIDE/

When I do a 'show asp drop' I see a huge amount of First TCP Packet not SYN ('tcp-not-syn') drops. I'm not sure whether these are being generated on the INSIDE or the OUTSIDE interface.

sh asp drop

Frame drop:

  Flow is denied by configured rule (acl-drop)                             95989

  First TCP packet not SYN (tcp-not-syn)                                  112941

  Bad TCP flags (bad-tcp-flags)                                                 18

  TCP data send after FIN (tcp-data-past-fin)                               2

  TCP failed 3 way handshake (tcp-3whs-failed)                          3280

  TCP RST/FIN out of order (tcp-rstfin-ooo)                                 14993

  TCP SYNACK on established conn (tcp-synack-ooo)                9

  TCP packet SEQ past window (tcp-seq-past-win)                     2340

  TCP invalid ACK (tcp-invalid-ack)                                            1

  TCP RST/SYN in window (tcp-rst-syn-in-win)                            23

  TCP packet failed PAWS test (tcp-paws-fail)                           2567

  FP L2 rule drop (l2_acl)                                                          240223

  Dropped pending packets in a closed socket (np-socket-closed)              285

Flow drop:

  Inspection failure (inspect-fail)                                                18

Any assistance would be greatly appreciated!!

Best Regards

Brett Verney

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